Abstract Gel permeation chromatography is one of the most powerful techniques for characterizing the polydispersity of polymeric materials. A versatile commercial apparatus has been used successfully in numerous laboratories on various problems of molecular weight distributions. But one of the difficulties still unsolved is the problem of calibration, i.e., the relation between elution volume and molecular weight. Some authors have assumed that retention time depends on the contour length of the molecular chain. Others think that it is more reasonable to use the radius of gyration or some average volume of the polymer molecule as the calibration parameter. In a recent paper we have reported GPC retention times of a series of polystyrenes exhibiting different molecular structures: linear, star-shaped, and comb-like. All were of known molecular weight and of low polydispersity. Obviously the conventional calibration method, where the logarithm of molecular weight is plotted against elution volume, does not yield a universal curve for all the samples. Retention times for branched samples were always larger than those for the linear homologs of the same molecular weight. We therefore tried using the hydrodynamic volume as the calibration parameter.